Composite structures are used in a wide variety of applications due to their high strength-to-weight ratio, improved corrosion resistance, and other favorable properties. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, and other components. For example, a wing panel may be produced by co-curing or co-bonding a plurality of elongated composite stringers to a composite wing skin. The process of fabricating each composite stringer may include forming a stack or charge of uncured composite plies over an elongated forming mandrel so that the composite charge assumes the shape of the forming mandrel, after which the composite charge is cured. Each composite ply may be comprised of reinforcing fibers (e.g., carbon fibers) impregnated with polymer matrix material (e.g., epoxy resin).
Conventional methods of forming an uncured composite charge over a forming mandrel may result in the occurrence of wrinkling or buckling of the reinforcing fibers. Such wrinkles may occur in the innermost composite plies at locations along inside corners formed in the composite charge. Localized wrinkling or buckling along inside corners of the composite charge may occur as a result of the buildup of compressive forces in the composite plies located nearest the forming mandrel. The buildup of compressive forces is due to the inability of adjacent composite plies to slip relative to one another as the composite charge is conformed to the cross-sectional shape of the forming mandrel. Wrinkles may also occur in any one or more of the composite plies of a partially-cured composite charge when formed into a contour that results in some areas of the composite charge being put into a compressive state. Areas of wrinkling in a composite structure may represent locally bent or kinked reinforcing fibers, and which reduces the strength and/or stiffness of the cured composite structure.
As can be seen, there exists a need in the art for a system and method for forming a composite charge over a forming mandrel without the occurrence of localized wrinkling or buckling of composite plies.